The field of art to which this invention pertains is compositions having more than one 1,2-epoxy group plus curing agents.
Dicyandiamide is a well known curing agent for epoxy resins. It is essentially non-reactive with epoxy resins at room temperature but breaks down at elevated temperatures to cure the resins. The mixture of epoxy resin and dicyandiamide must be heated to a temperature of about 165.degree. C. and held at this temperature for an extended period of time to effect a cure. The rate of cure increases with temperature. Various catalyst, e.g., tertiary amines, have been used with dicyandiamide to lower the curing temperature and to increase the cure rate. In U.S. Pat. No. 2,928,811, quaternary ammonium salts are disclosed as being useful accelerators for epoxy resin-dicyandiamide reactions. Other accelerators are acyl guanidines and metallic salts of polyamines, disclosed, respectively, in U.S. Pat. Nos. 3,397,156 and 3,397,157.
In some instances, particularly on production lines, the cure rate of epoxy resins with dicyandiamide with no accelerator is too slow for efficient operation, whereas the accelerated curing systems are too fast. Insufficient time is allowed to adjust resin flow and substrate wetting.
U.S. Pat. No. 3,079,418 discloses compositions made from haloalkyl phosphoric acid esters in admixture with epoxy resins. The epoxy resin is present in the composition in the amount of 0.1 to 10 percent by weight and serves as a stabilizer against decomposition of the haloalkyl phosphoric acid ester. U.S. Pat. No. 3,477,982 describes the use of red phosphorus in epoxy resin compositions for flame proofing. In U.S. Pat. No. 3,372,208, flame resistant epoxy resins are made by reacting an epoxy resin with a phosphorus trihalide. None of these references disclose curing systems made by adding phosphorus compounds to an epoxy resin-dicyandiamide-tertiary amine composition.